1. Field of the Invention
The present invention relates to a co-channel communication system for concurrently transmitting a data signal with a video signal over a single communications channel and more particularly, to a communication system for transmitting data in an active or visible portion of a video raster in a manner that is imperceptible to a viewer.
2. Description of the Prior Art
Various communication systems are known in the art for transmitting data signals along with a video signal. Such data signals are known to be used for many different purposes, for example, program syndication verification, closed caption data and the like. These communication systems utilize various methods for transmitting such data along with the video signal. In some known systems, the data is transmitted in inactive portions of the video raster, such as during vertical blanking intervals. However, due to the increased demand for additional data, non-active portions of the video rasters are generally no longer available for data transmission. Other known systems place the data in an active portion of the video raster by modulating the data at low levels relative to the video signal. More specifically, in such systems, the data signal is normally transmitted over an active video line that appears near the top or bottom of the screen since a viewer is less likely to concentrate on a portion of the display away from the center of the screen. However, such data signals appear as noise and are still perceptible to the viewer.
Various other techniques are known for transmitting data signals along with a video signal For example, U.S. Pat. No. 3,244,808 to Roberts discloses a pulse code modulation (PCM) video communication system. In such systems, digital data is used to improve the picture quality. More specifically, in such a system, the picture quality is related to the number of bits utilized to encode the brightness level. In order to improve the picture quality in such a system while minimizing the channel bandwidth, a data signal is used to average the brightness level in a selected area of the video raster to reduce the contrast and thereby improve the picture quality. The data signal is added to the video signal by way of a pseudo-random sequence at the transmitter. The same pseudo-random sequence is used at the receiver in order to decode the data signal.
Spread spectrum techniques are also known for transmitting data over active video channels More specifically, U.S. Pat. No. 4,912,721 to Pigeon et al. discloses an interactive video communication system which utilizes spread spectrum signals for sending data from a cable TV household to a cable head end. More specifically, in such a communication system a return path is provided to allow a cable TV subscriber to communicate with a cable head end for a variety of purposes such as home shopping, voting, and the like.
U.S. Pat. No. 4,425,642 to Moses et al. discloses a co-channel communication system for transmitting a digital data signal simultaneously with a video or voice signal. The Moses et al. patent discloses the use of spread spectrum signals to transmit the digital data signal along a predetermined active video line. Although the use of spread spectrum signals greatly reduces the viewer perceptibility of the data signal, transmission of the data signal over a single active video line is still noticeable to a viewer